Bistable nematic liquid crystal display devices generally require large AC electric potentials to initiate interstate switching between bistable states. One important reason for such large AC electric switching potentials is that sufficient electric energy must be supplied to each display cell for detaching and moving disclinations from pinning sites.
One embodiment of a nematic liquid crystal display device exhibits configurational bistability between two states. See U.S. Pat. No. 4,333,708 issued to G. D. Boyd et al. on June 8, 1982. The two states, which exist separately in the absence of a holding potential, are topologically inequivalent and derive stability from disclination pinning. Interstate switching is accomplished by detaching and moving disclinations from a pinning site in response to an applied AC switching potential which exceeds a large, sharp switching threshold.
Reduction of the switching threshold level for this type of liquid crystal display device is achieved by prebiasing selected cells in the display with a small AC priming potential prior to applying the larger switching potential. See, for example, copending U.S. patent application, Ser. No. 411,305, filed Aug. 25, 1982. Although the switching level is lower for the latter nematic liquid crystal display device, the switching level remains sufficiently high, for example, on the order of one hundred volts, to ensure detachment and motion of the disclinations necessary to achieve interstate switching.
It should be noted with respect to both of the display devices described above that AC switching potentials are employed to effect interstate switching. The signals which generate these AC switching potentials are generally from the family of constant envelope signals and, more particularly, of substantially constant envelope, gated, AC pulse signals. Constant envelope AC signals are preferred to constant amplitude or DC signals because the latter signals give rise to space charge polarization effects which reduce the amplitude of the applied electric field.
For both of the above display devices, the problems of relatively large AC switching potentials and switching by disclination motion still exist.